Influenza has been raging around the world. Surprisingly, according to an article posted on The SUN (25th January 2018), "Japanese Flu" has been prevalent in the UK. It is caused by the B type Yamagata strain virus, which was confirmed in the Yamagata prefecture in 1988.

In 1999, Japan established a system of 5,000 fixed-point influenza observation medical institutions throughout the country and estimated the number of patients nationwide on a weekly basis starting in September, when flu season commences. According to a report entitled "About the occurrence of influenza" issued on February 9, 2018 by the Ministry of Health, Labor and Welfare, the cumulative number of patients in the period from September, 2017 to the fifth week of 2018 (from January 29 to February 4) was 13,930,000, the highest ever recorded. As shown in Figure 1, the number of patients has been increasing rapidly since the third week of 2018.